182 



NA TURE 



[August 5, 192: 



Electricity and Matter. 1 



By Sir Ernest Rutherford, F.R.S. 



IT has been customary in the earlier Kelvin lectures 

 to give an account of some phase of Kelvin's 

 work. 1 could easily follow this custom by concentrat- 

 ing on the publications of Kelvin that deal with the 

 pn mi 11I the atomic nature of matter and the dimensions 

 of atoms and molecules, including the first suggestions 

 of the mechanism of atomic constitution. This was 

 a subject in which Kelvin was permanently interested. 

 In his Royal Institution lecture of 1883, reprinted in 

 " Popular Lectures and Addresses," vol. 1, he gives 

 an illuminating account of the different lines of evidence 

 that all converge to a cumulative proof that matter 

 is coarse-grained or atomic in structure and set a 

 definite minimum limit to the dimensions of the atom. 

 His deduction of the diameter of the water molecule 

 from the cooling effect observed when a water film is 

 stretched, is one of the most notable of these examples. 

 In his later papers he accepts Stoney's arguments in 

 support of the atomic nature of electricity, and in a 

 paper of curious title, " Jipinus Atomised," 2 he re- 

 states the old theory of ..Epinus of the nature and 

 relation of positive and negative electricity in a more 

 modern form, by assuming that the negative electricity 

 in an atom is distributed in the form of definite units 

 called " electrions " — or electrons, as we should now 

 term them — held in equilibrium embedded in a sphere 

 of uniform positive electrification. This was the first 

 type of model atom put forward. A similar type of 

 atom, developed and worked out in detail by Sir J. J. 

 Thomson, played a notable part in giving a concrete 

 view of atomic structure which was directly amenable 

 to mathematical calculation. In some of his later 

 papers. Kelvin devised types of atoms which, under 

 certain disturbances, broke up with explosive violence, 

 simulating in behaviour the atoms of radium. While 

 keenlv interested in such speculations, there remained 

 the curious anomaly that he did not accept entirely 

 the current explanation that radio-activity was a 

 consequence of the successive disintegrations of atoms. 



The discovery in 1897 of the individual existence 

 of the negative electron of small mass, and the proof 

 that it was a component of all the atoms of matter, 

 was an event of extraordinary significance to science, 

 not only for the light which it threw on the nature of 

 electricity, but also for the promise it gave of methods 

 of direct attack on the problem of the structure of the 

 atom. This discovery of the electron, coupled with 

 the recognition of the atomic nature of electricity, has 

 created a veritable revolution in our ideas of atoms. 



The first definite proof of the close relations that 

 exist between electricity and matter we owe to the 

 famous experiments of Faraday on the passage of 

 electricity through electrolytes. It was clear that 

 the simple numerical relations found by him between 

 the electrochemical equivalents of the elements and 

 their atomic weights could be simply interpreted by 

 assuming that electricity was atomic in character 

 and that the charges carried by the individual ions 

 were integral multiples of a fundamental unit of charge. 

 It is curious to note the long interval that elapsed 



1 From the thirteenth Kelvin lecture, delivered before the Institution of 

 Electrical Engineers on May 18. 



- PhiloS'if'hu 11I Mii£ii-inr, March 190^. 



NO. 2753, VOL. I IO] 



before the idea of the atomic nature of electricity was 

 generally accepted by men of science — possibly because 

 of the great difficulty of obtaining confirmatory 

 evidence. The suggestion was mentioned by Maxwell 

 and' Helmholtz, although with reservation, but was 

 revived with conviction by Johnstone Stoney, who 

 suggested that the name " electron " should be applied 

 to the fundamental unit of electricity and made a 

 rough estimate of its magnitude. Actually, as we 

 know, the term " electron " is now used to denote 

 not the actual value of the unit of charge, but the 

 free atom of negative electricity. 



Following the discovery of the independent existence 

 of the electron and the proof of the production of 

 charged ions in gases by X-rays and other radiations, 

 it was implicitly assumed by men of science that 

 electricity must be atomic in nature, and all the 

 experimental data were interpreted on this view. It 

 was found by Townsend that the charge carried by 

 the ions produced in gases and by the electron itself 

 was numerically equal to that carried by the hydrogen 

 ion in the electrolysis of water, which was taken as 

 the fundamental unit. By the ingenious device of 

 balancing the weight of a charged drop by the attraction 

 of the electric field, Millikan was able to give a very- 

 direct and convincing proof of the correctness of this 

 view and to determine the magnitude of the funda- 

 mental unit with great precision. Knowing the value 

 of this constant, the electrochemical data give us 

 immediately the mass of the atom of each of the 

 elements. While no one now doubts the atomic 

 nature both of matter and of electricity, it should be 

 noted that the atomic nature of matter is in reality 

 a consequence of the discrete nature of electricity, for 

 all the evidence indicates that the atom itself is a 

 purely electrical structure. 



It was soon recognised that the negative electron of 

 small mass was an actual disembodied atom of elec- 

 tricity, and that its apparent mass was electrical in 

 origin. Sir J. J. Thomson had shown as early as 1881 

 that a charged body in motion behaved as if it had 

 an additional electric mass due to its motion. The 

 moving charge generates a magnetic field in the space 

 surrounding it, resulting in an increase of energy of 

 the moving system which is equivalent to the effect 

 produced by an increase of the mass of the body. The 

 experiments of Kaufmann and others on the swift 

 electrons ejected from radium showed that the mass 

 of the electron, while sensibly constant for slow fields, 

 increased rapidly as the velocity of the electron ap- 

 proached that of light. This variation of mass was 

 in good agreement with calculations based on the 

 electrical theory. Later, Einstein from considerations 

 of relativity snowed that for any material particle, 

 whether charged or not, the mass m must vary with 

 speed according to the relation mjin u = (i -/& 2 ) - *, 

 where wj is the mass for low speeds, and /i is the ratio 

 of the velocity of the particle to the velocity of light. 

 Experimental results agree closely with this calculation. 



Since there must always be electric mass associated 

 with the movement of electric charges, it is natural 



